The boyfriend of Stuart Rattle killed the interior designer and then maintained a charade of normality for five days before trying to conceal the murder in a fire, according to court documents.

Michael Anthony O'Neill, 48, on Tuesday admitted to killing Mr Rattle, 53, in December 2013 and pleaded guilty to charges of murder and arson in Melbourne Magistrates Court.

"Yes, Your Honour, I am. I am guilty, Your Honour," O'Neill told magistrate Gerard Lethbridge when asked for a plea, as relatives of both men watched on.

Mr Rattle's body was found in the couple's burnt-out South Yarra home on December 8. But, according to a prosecution summary tendered to court, police believe O'Neill killed his partner of 16 years up to five days earlier.

According to the summary, O'Neill took a saucepan from the kitchen about 6am on December 4 and returned to the bedroom and hit Mr Rattle over the head. He then strangled him with a nylon dog lead.

O'Neill then went through an "elaborate pretence" over the next five days to pretend everything was normal, according to the summary prepared by Chief Crown prosecutor Gavin Silbert, QC.

He maintained social and business appointments, used Mr Rattle's phone to pose as his partner and claim the designer was unwell and in bed, and visited the couple's Musk farm near Daylesford, Mr Silbert's summary says.

"The only change to his routine seems to be that he slept on the sofa rather than in the bed with the dead man," the summary says.

O'Neill set fire to the apartment on December 8, the prosecution says, and then told police Mr Rattle had been trapped in the fire. But autopsy results found there was no soot in Mr Rattle's airways and that there was no evidence to suggest he was alive before the fire started.

O'Neill's barrister, Ruth Shann, told the court on Tuesday her client was in a "fragile state of mind" and had lost 20 kilograms since his arrest. She applied to have O'Neill's record of interview with police suppressed from the media in court because his "intense distress is apparent" in the interview. She said there was also an absence of an explanation for why O'Neill "spontaneously reacted as he did".

Mr Lethbridge rejected her application.

O'Neill, dressed in a navy blue suit and light blue shirt, was remanded in custody to appear before the Supreme Court on September 17.